Method for manufacturing solder mask of printed circuit board

ABSTRACT

A solder mask manufacturing method adapted to apply a solder mask on a surface of a substrate of a circuit board, said surface is provided with a conductor pattern having an unsheltered portion and a sheltered portion which is covered by said solder mask. The method comprises the steps of: a) disposing a layer of semi-solid solder mask material having an expansion coefficient substantially the same as that of the substrate on the surface of said substrate to cover said copper conductor pattern, and a metal foil covering the material layer; b) applying pressure to the metal foil and applying baking treatment to cure the solder mask material in to solid; c) utilizing chemical solution and plasma etching to remove the metal foil and the solid solder mask material above the unsheltered portion of said copper conductor pattern respectively such that the unsheltered portion can be exposed; and d) using chemical solution to remove the residual metal foil.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a method formanufacturing a printed circuit board (PCB), and more particularly to amethod for manufacturing the solder mask of the printed circuit board.

BACKGROUND OF THE INVENTION

[0002] In the manufacture of a conventional printed circuit board, afterforming the outer layer conductor pattern, a solder mask protectivetreatment has to be applied to the conductor pattern so as to preventoxidation of the conductor pattern or solder connection short circuit.

[0003] In a solder mask manufacturing process for the conventionalprinted circuit board, a solder mask paint is applied to the surface ofthe circuit board by halftone printing, roller type coating, screencoating or electrostatic spray coating, etc. After pre-baking, dryingand cooling, exposure is employed to display images. The method ofimaging removes unnecessary paint, and the resin in the remaining paintis completely cured by high-temperature baking in the final step to forma solder mask covered on the surface of the PCB to protect the conductorpattern.

[0004] The conventional solder mask manufacturing process has thefollowing disadvantages:

[0005] 1. Using halftone printing or roller type coating to apply thesolder mask paint to the printed circuit board requires multiplehalftone printing or roller printing steps in order to accumulate thepaint to the desired thickness, which is complicated.

[0006] 2. During the process of baking the printing circuit board athigh temperature, since the resin material in the solder mask paint hasan expansion coefficient different from that of the circuit boardsubstrate, internal stress will form between the resin material and thecircuit board to cause the bend of the printed circuit board.

[0007] 3. The thickness of the solder mask paint layer is not easy tocontrol, resulting in electric instability of the circuit board.

[0008] 4. Air bubbles can easily get in during the process of coatingthe solder mask paint to result in poor reliability quality of thesolder mask and shortened life of the solder mask layer.

[0009] 5. Coating the circuit board with solder mask paint cannot fillthrough holes in the circuit board with solder mask paint. Therefore,before coating, insulating fillers are disposed in the through holesrespectively and are ground flat before coating the solder mask paint,thereby increasing manufacturing time and cost of the printing circuitboard.

[0010] 6. When coating the circuit board with solder mask paint, coatinglayer cannot be easily formed on the copper foil conductor pattern, andthe bonding therebetween is relatively poor.

SUMMARY OF THE INVENTION

[0011] The primary object of the present invention is to provide asolder mask manufacturing method for a printed circuit board that hasgood quality and that will not cause bend of the circuit board.

[0012] In order to achieve the aforesaid object, the present inventionis to provide a solder mask manufacturing method adapted to apply asolder mask on a surface of a substrate of a circuit board, said surfaceis provided with a conductor pattern having an unsheltered portion and asheltered portion which is covered by said solder mask. The methodcomprises the steps of: disposing a layer of semi-solid solder maskresin material having an expansion coefficient substantially the same asthat of the substrate on the surface of said substrate to cover saidcopper conductor pattern, and a metal foil for covering the resinmaterial layer; applying predetermined pressure to the metal foil andbaking at a predetermined temperature for a predetermined time to curethe semi-solid solder mask resin material; utilizing chemical solutionetching to remove the metal foil above the unsheltered portion of saidcopper conductor pattern, and using plasma etching to remove the solidsolder mask material above said unsheltered portion of said conductorpattern such that the unsheltered portion of the said conductor patterncan be exposed; and lastly, using chemical solution to remove theresidual metal foil.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a flow chart of a preferred embodiment of the presentinvention; and

[0014]FIG. 2 is a schematic view of the preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring to FIG. 1, the solder mask manufacturing method for aprinted circuit board of the present invention adapted to apply a soldermask on a surface of a substrate of the printed circuit board, saidsurface of the substrate is provided with a cooper conductor patternhaving an unsheltered portion and a sheltered portion which is coveredby said solder mask. The method includes the following steps:

[0016] 1. Coating of solder mask resin material: disposing a layer ofsemi-solid solder mask resin material of a predetermined thickness andexpansion coefficient identical or similar to that of the circuit boardsubstrate on the surface of the substrate such that the cooper conductorpattern is covered by said resin material, and then a metal foil of apredetermined thickness is disposed to cover the solder mask resinmaterial.

[0017] 2. Press bonding and curing of solder mask resin material:applying predetermined pressure to the metal foil such that solder maskresin material between the metal foil and the substrate of the printedcircuit board tightly covers the surface of the substrate, and thenbaking treatment is applied at a predetermined temperature for apredetermined time to cure the semi-solid solder mask resin material.

[0018] 3. Removing unnecessary solder mask material: utilizing chemicalsolution etching to remove the metal foil above the unsheltered portionof the cooper conductor pattern, and then using plasma etching to removethe solder mask resin material above the unsheltered portion of thecooper conductor pattern such that said unsheltered portion of thecopper unsheltered portion can be exposed.

[0019] 4. Removal of metal foil: using chemical solution to remove theresidual metal foil covering the solder mask material, therebycompleting the manufacturing of the solder mask.

[0020] The aforesaid is the steps of the process of this invention.Hereinafter, the invention is described in detail using a preferredembodiment with reference to FIG. 2.

[0021] Referring to FIG. 2, in the solder mask manufacturing process fora printed circuit board of this invention, a printed circuit board 1which has undergone the steps of hole drilling and plating, and outerlayer cooper conductor pattern circuit layout is subjected to soldermask treatment of the outer layer conductor pattern of the circuit board1. The material of the substrate 11 of the printed circuit board 1 ismulti-function epoxy resin. Therefore, in this embodiment, the printedcircuit board 1 has a plurality of through holes 12 and predeterminedouter layer copper conductor pattern 13 (see FIG. 2A). The cooperconductor pattern 13 can be defined into two portions, namely, theunsheltered portion 13 a and the sheltered portion. The solder maskmanufacturing process for the printed circuit board 1 is as follows:

[0022] 1. Coating of Solder Mask Material:

[0023] Firstly, prepare a aluminum foil 2 with a thickness between 20μm˜40 μm. Apply solder mask resin material 3 of a thickness between 30μm˜100 μm to one side of the aluminum foil 2. The solder mask materialis made of multifunction epoxy resin paint identical to that of thesubstrate 11 of the printed circuit board 1 with organic solution added,and is pre-baked to form a semi-solid state (i.e. thick and sticky).Cover the side of the circuit board 1 that is to be solder mask treatedwith the solder mask resin material on the aluminum foil 2 so that thesolder mask material 3 is sandwiched between the substrate 11 of thecircuit board 1 and the aluminum foil 2 (see FIG. 2B).

[0024] 2. Press Bonding and Curing of Solder Mask Material:

[0025] Apply uniform force of 10˜40 kgw/cm² to the aluminum foil 2, andbake the solder mask material 3 at a temperature of 185° C. for 1.5hours to 3 hours such that:

[0026] a) The solder mask material 3 fills through holes 12 andmicro-pores such as blind holes in the substrate 11 of the circuit board1 due to pressure applied and tightly covers the substrate 11 to form acover layer.

[0027] b) Organic solution in the solder mask material 3 evaporates whenheated, and epoxy resin in the solder mask material 3 gradually cures toform a solder mask protective layer.

[0028] After curing of epoxy resin in the solder mask material 3 to forma solder mask protective layer, release the pressure and temperatureapplied.

[0029] 3. Removal of Unnecessary Solder Mask Material:

[0030] Press a dry film photo-resist 4 on the aluminum foil 2 withsuitable temperature and pressure (as shown in FIG. 2C), and subject thedry film photo-resist 4 to light exposure in conjunction with a negativefilm so that the dry film photo-resist 4 generates images relative topositions of the sheltered portion of the cooper conductor pattern 13.Etch the parts of the dry film 4 a without images and the aluminum foil2 a using carbonic acid and ferric chloride (FeCi₃) solution withspecific gravity 1.3˜1.5 and temperature of 40° C.˜60° C. so that epoxyresin solder mask layer at where the aluminum foil was removed isexposed on the outside (as shown in FIG. 2D), and then remove theresidual dry film 4 using sodium hydroxide of 1˜3 wt % and temperatureof 45° C.˜65° C.

[0031] Subsequently, plasma etching is used to remove the exposed epoxyresin solder mask layer so that the unsheltered portion 13 a of thecooper conductor pattern is exposed on the substrate 11 (as shown inFIG. 2E).

[0032] 4. Removal of Metal Foil:

[0033] Lastly, use phosphoric acid (H₃PO₄) solution of 60%˜80% volume tovolume and temperature of 50° C.˜80° C., or hydrochloric acid (HCl) withconcentration of 10%˜40% and temperature of 20° C.˜40° C. to remove theresidual aluminum foil 2 (as shown in FIG. 2F), completing the entiresolder mask manufacturing process. The printed circuit board 1 cansubsequently undergo nickel-plating operation of the unsheltered portion13 a of the cooper conductor pattern 13. Since phosphoric acid orhydrochloric acid can corrode aluminum and will not corrode copperseverely, this is the reason why this embodiment selects aluminum foilas cover layer of the solder mask material.

[0034] As techniques relating to pressing the dry film, light exposure,imaging and plasma etching are known art, it is not necessary to furtherdescribe the same herein.

[0035] Hereinafter, the advantages of the embodiment are set forth asfollows:

[0036] 1. The thickness of the solder mask 3 can be set at one time toreduce process step so as to reduce process costs.

[0037] 2. Since the solder mask 3 and substrate 11 of the circuit board1 are of the same multifunction epoxy resin, the expansion coefficientthereof is the same as that of substrate 11 of the circuit board 1.Therefore, during the process of baking in step 2, internal stress willnot be present between the solder mask 3 and the substrate 11 so as notto cause bend of the circuit board 1. Particularly, application ofuniform force to the substrate 11 during the process of baking in step 2will prevent deformation of the circuit board 1.

[0038] 3. As the solder mask material 3 is attached to the substrate 11,it is subjected to an uniform force via the aluminum foil 2 for a periodof time, the solder mask material 3 will cover the circuit board 1 withuniform thickness and forms a solder mask protective layer with uniformthickness after curing so that the circuit board 1 has stable electriccharacteristics.

[0039] 4. As the solder mask material 3 is attached to the substrate 11,it is subjected to an uniform force via the aluminum foil 2 until cured,air bubbles will not get thereinto during that period of time and cantherefore enhance quality of the solder mask protective layer andprolong the life thereof

[0040] 5. Due to the pressure applied, the solder mask material 3 canfill through holes and micro-pores such as blind holes in the substrate11 simultaneously. Therefore, it is not necessary to fill the throughholes with insulating filler material and grind the same even prior tosolder mask of the circuit board 1, thereby reducing manufacturing timeand costs of the printed circuit board.

[0041] 6. As the solder mask material 3 is caused to cover the surfaceof the substrate 11 by applying pressure and heating, copper conductorpattern 13 on the substrate 11 can easily attach thereto and will notdetach therefrom.

[0042] Although the present invention has been illustrated and describedwith reference to the preferred embodiment thereof, it should beunderstood that it is in no way limited to the details of suchembodiment but is capable of numerous modifications within the scope ofthe appended claims.

What is claimed is:
 1. A printed circuit board comprising: a substratehaving two opposite surfaces, two conductor patterns respectivelydisposed on the surfaces of said substrate, and at least one platedthrough hole electrically interconnected with said conductor patterns,each of said conductor patterns having an unsheltered portion and asheltered portion; two solder mask layers respectively coated on thesurfaces of said substrate and covered over the sheltered portions ofsaid conductor patterns and filling the at least one plated through holesuch that the unsheltered portions of said conductor patterns areexposed outside; wherein each of said solder mask layers has a thermalexpansion coefficient substantially identical to that of said substrate;2. The printed circuit board as defined in claim 1, wherein saidsubstrate is made of a material containing a resin, and each said soldermask layer is made of a material containing a resin which issubstantially identical to the resin of said substrate.
 3. The printedcircuit board as defined in claim 2, wherein the resin contained in thesubstrate is epoxy resin.
 4. The printed circuit board as defined inclaim 1 further comprising two metal foils respectively covered on saidsolder mask layers in the way that the unsheltered portions of saidconductor patterns are exposed outside.